A kind of odd spec showed up as top for threat in Unholy tanking, with no [spell]reaping[/spell], no [spell]epidemic[/spell] and [spell]ravenous dead[/spell]. This is in simulations only, no in-game testing. I haven’t had any corroboration yet, so watch this space.
That old chestnut of [spell]Death and Decay[/spell] single-target rotation does even better, if you want to use it glyphed with T10 bonus. Its playstyle just feels weird to me though.
On this basis, Unholy’s threat is comparable to DW Frost, about 5% behind.
All my usual caveats apply to Unholy: bone shield uptime, no physical damage reducer, sims have limitations and possible bugs too, etc.